The present invention generally relates to a black level clamping circuit used in a high definition color CRT display and a high quality television receiver to be used in a computer terminal.
At present, a color CRT adopts in-line electron guns. Since three electron guns of red, blue, green become common except for a cathode electrode, the respective cathode voltages have to be properly selected so as to arrange the level of the beam currents of the three electron guns. If the voltage difference thereof is put together with the voltage difference for changing the difference of the brightness, the variable range of the cathode voltage is required to be 40V through 60V. In the general television receiver, the frequency band of the picture signal is narrow (6 MHz), so that the cathode voltage may be changed by the DC coupling of the picture output circuit to the cathode electrode to change the input bias voltage.
But in the high definition color CRT display, the frequency band of the picture signal is wide (20 to 30 MHz), so that a similar DC insertion circuit cannot be used, but a keyed clamping circuit (or pulse clamping circuit ) must be used.
FIG. 1 shows an example of a conventionally used keyed clamping circuit.
Referring now to the drawings, there is shown in FIG. 1, a conventional clamping circuit, which includes a drive transistor 1, with the picture signals being inputted into the base of the drive transistor, an output transistor 2, a by-pass capacitor 3 connected to the output transistor 2, resistors 4 and 5 which divide the power supply voltage Vo to feed the by-pass voltage to the output transistor 2, a load resistor 6, a peaking coil 7, thereby constituting one color of a picture output circuit, and a coupling capacitor 8 for applying picture signals to a CRT cathode 9, a transistor 10 for amplifying the clamping pulses, a transformer 11 for feeding the clamping pulses to the transistor 10, a load resistor 12 of the transistor 10, a diode 13 for applying the output of the transistor 10 to the CRT cathode 9, a variable resistor 14 for varying the output of the transistor 10 so as to constitute a keyed clamping circuit with the transistor 10, the transformer 11, the resistor 12, the diode 13, and the variable resistor 14.
The operation of the clamping circuit made up as described hereinabove will be described with reference to FIGS. 3(a)-3(e) showing the voltage waveforms of each section. When the input picture signal is a step wave signal of a period T as shown in FIG. 3(a), the picture output signal is as shown in FIG. 3(b), wherein VO is a power supply voltage, VB is a block level, and Vw is a white level. The blanking pulse shown in FIG. 3(c) is made in the horizontal output circuit on the normal display at the picture blanking period from a time to to t1. When the pulse is applied to the transformer 11 as the clamping pulse, it is amplified so as to generate the output voltage shown in FIG. 3(d) at the collector of the transistor 10. Since the voltage V1 is lower than the black level VB, the diode 13 becomes conductive at the picture blanking period, the coupling capacitor 8 is charged, so that an electric charge proportional to (VB - V1) is ideally accumulated. After the picture blanking period has been completed, the diode 13 becomes reverse biased, and the coupling capacitor 8 is cut off from the keyed clamping circuit. The voltage of the CRT cathode 9 becomes the voltage shown in FIG. 3(e) due to the remaining electric charge, so that the DC voltage shift is effected. Since the gain of the keyed clamping circuit is almost determined by the ratio in the resistance value of the resistor 12 and the variable resistor 14, the voltage V1 may be optionally changed by the value change in the variable resistor 14, so that the difference in the beam current at the black level among the three electron guns may be removed. As the V1 becomes higher when the input voltage of the transformer 11 is made smaller, the brightness becomes lower so that the brightness may be optionally selected.
When the CRT is made larger in size, the beam current is required to be made large. As the impedance of the CRT cathode becomes lower when the beam current is made large, inconveniences are caused in the conventional black level clamping circuit.
In the circuit of FIG. 1, the impedance of the CRT cathode 9 when the picture signal is the black signal only becomes infinite as the beam current does not flow. The coupling capacitor 8 is not discharged. After some discharging operations, the voltage across the capacitor 8 becomes (VB-V1) as described hereinabove. But as the impedance of the CRT cathode 9 becomes 50K.OMEGA. through 100K.OMEGA. when the beam current flows at 1 mA in the case of the white signal only, the coupling capacitor 8 is charged more than when the beam current is less, so that the charging operation cannot catch up with the discharging operation. The voltage across the capacitor of becomes too high to be neglected as compared with that of the lower beam current. Namely, there is a problem in that the DC voltage of the signal becomes higher in the cathode 9 and brightness cannot be correctly reproduced.